In recent years, as fuel cells for automobiles, polymer electrolyte fuel cells have started to progress rapidly. The polymer electrolyte fuel cell is a fuel cell that uses hydrogen and oxygen, and also uses an organic film (composites with inorganic materials are also being developed) of a hydrogen-ion-selective-transmission type as electrolyte. Examples of hydrogen used as a fuel include pure hydrogen and a hydrogen gas obtained by modifying alcohols.
However, current fuel cell systems use components and members having high unit costs, and the cost for components and members needs to be lowered largely for consumer use. Further, for use in automobiles, the current fuel cell systems need not only to lower the cost, but also to downsize a stack, which serves as the center of the fuel cell. A polymer electrolyte fuel cell has a structure in which a membrane electrode assembly (hereinafter also referred to as MEA) including a solid polymer film, electrodes, and a gas diffusion layer, is sandwiched between separators, and a large number of MEAs are laminated to form a stack.
Examples of characteristics required for the separator include electron conductivity, a property of isolating an oxygen gas and a hydrogen gas at the respective electrodes, low contact resistance with the MEA, favorable durability in the environment inside a fuel cell, and the like. Here, the gas diffusion layer (GDL) in the MEA is generally formed with a carbon paper consisting of integrated carbon fibers, and accordingly, the separator is required to have favorable contact conductivity with carbon.
Since a stainless steel, a titanium material, and the like, which is used as materials for a separator, generally has low contact conductivity with carbon without any treatment, many techniques have been proposed to increase the contact conductivity with carbon. A passivation film having low conductivity can serve as an obstacle to higher contact conductivity with carbon. Although this problem could be solved at the expense of the durability, extremely high durability is still required for a separator in the environment inside the fuel cell, which is a highly corrosive environment.
For this reason, currently, it is quite difficult to develop a satisfactory metal material for a separator. Carbon separators have been the mainstream so far; however, if meal separators become available, the fuel cell itself can be downsized, and further, a break will not occur in the manufacturing process of the fuel cell. Accordingly, metal separators are strongly demanded to enable mass production and diffusion.
Under such circumstances, for example, Patent Document 1 discloses a technique that makes it possible to lower contact resistance of a stainless steel effectively, in terms of thinning, reducing weight, and the like, by use of a special stainless steel obtained by precipitating a conductive compound in a steel material.
Highly durable titanium is also being studied to be used for a separator. In the same manner as a stainless steel, titanium has high contact resistance with the MEA by the presence of a passivation film on the outermost surface of titanium, and accordingly, for example, Patent Document 2 discloses a technique that enables a TiB-based precipitate to be diffused in titanium and the contact resistance with the MEA to be lowered.
Patent Document 3 discloses a titanium alloy for a separator. The titanium alloy contains, by mass %, 0.5 to 15% Ta and a limited amount of Fe and O as necessary. Further, in the titanium alloy, a range from the outermost surface to 0.5 μm in depth has an average nitrogen concentration of greater than or equal to 6 atomic %, and contains tantalum nitride and titanium nitride.
Patent Document 3 also discloses that, in a method for manufacturing a titanium alloy for a separator, it is preferable to heat the titanium alloy at temperatures of 600 to 1000° C. for three seconds or more under a nitrogen atmosphere.
Patent Documents 4, 5, and 6 disclose a technique to thrust a conductive material into the superficial layer by a blasting method or a roll processing method in a manufacturing process of a titanium or stainless steel metal separator. In this technique, a surface microstructure in which the conductive material is disposed to penetrate a passivation film formed on the metal surface secures both contact conductivity with carbon and durability.
Patent Document 7 discloses a method for manufacturing a fuel cell separator, including converting impurities including titanium carbide or titanium nitride formed on the surface of titanium into oxide by anode oxidizing treatment, and then performing plating treatment. Titanium carbide or titanium nitride formed on the surface of titanium is dissolved while being exposed to a corrosive environment and is re-precipitated as oxide that inhibits contact conductivity to lower the contact conductivity.
The above method suppresses oxidation of these impurities during generation of electricity (during use) and increases durability. However, to secure conductivity and durability, an expensive plated film is necessary.
Patent Document 8 discloses a technique to form an oxide film as a corrosion-resistant film by coating the surface of a titanium-based alloy with BN powder and by performing heat treatment thereon, the titanium-based alloy being used as a base material and being obtained by alloying Group 3 elements in the periodic table.
This is a technique to increase conductivity by doping, with impurity atoms, a position of a titanium atom in a crystal lattice of the oxide film serving as a passivation film of the titanium alloy.
Patent Documents 9 and 10 disclose a technique to form, in rolling processing of a fuel cell separator made of titanium, an altered layer containing titanium carbide on the superficial layer by rolling using carbon-containing rolling oil, and to form a high-density carbon film thereon to secure conductivity and durability.
In this technique, although conductivity with a carbon paper is increased, since durability is maintained by the carbon film, a fine carbon film needs to be formed. The interface between simple carbon and titanium has high contact resistance, and accordingly, titanium carbide that increases conductivity is disposed therebetween. However, in a case in which the carbon film has a defect, the altered layer (including titanium carbide) and the base material cannot be prevented from being corroded, and a corrosion product that inhibits contact conductivity may be generated.
Patent Documents 11, 12, 13, 14, and 15 disclose a titanium fuel cell separator that has a structure similar to that disclosed in Patent Document 9, which is a structure mainly including a carbon layer, a titanium carbide intermediate layer, and a titanium base material in this order. Although a manufacturing process is different from that in Patent Document 9 in that the titanium carbide intermediate layer is formed after the carbon layer is formed in advance, a mechanism of increasing durability by the carbon layer is similar.
Patent Document 16 discloses a technique to apply graphite powder to perform rolling and annealing for mass production. This technique enables the function of a conventional carbon separator by adding the carbon layer and the titanium carbide intermediate layer to the surface of an unbreakable titanium base material. However, since the titanium carbide intermediate layer has low durability, there remains a concern that this surface structure can generate a corrosion product that inhibits contact conductivity because the titanium carbide intermediate layer and the base material cannot be prevented from being corroded in a case in which the carbon film has a defect.
Under such circumstances, Patent Document 17 discloses a technique to dispose titanium carbide or titanium nitride, which is a conductive material, on the surface of titanium, and to cover not only titanium but also the conductive materials with titanium oxide having a passivation function.
This technique secures contact conductivity, and in addition, increases durability. However, it is necessary to further increase environmental deterioration resistance of the titanium oxide film covering the conductive materials in order to further lengthen the lifetime of the fuel cell.
Accordingly, the present applicants proposed, in Patent Document 18, a titanium or titanium alloy material for a fuel cell separator having high contact conductivity with carbon. This technique mainly increases durability by performing passivation treatment on a titanium oxide film in which the titanium oxide film is immersed in an aqueous solution containing an oxidizing agent such as nitric acid or chromic acid. In this technique, in addition, titanium compound particles including carbon or nitrogen that is a fine conductive material are dispersed in an oxide film on the surface of the titanium or titanium alloy material.
Patent Document 19 proposes to perform stabilization treatment after the passivation treatment in an aqueous solution using carbide, nitride, carbonitride, or boride of tantalum, titanium, vanadium, zirconium, or chromium as the fine conductive material. This stabilization treatment uses an aqueous solution including rice flour, wheat flour, potato starch, corn flour, soy flour, a pickling corrosion inhibitor, and the like, which are natural products and artificial synthetic substances containing one or more selected from amine-based compounds, aminocarboxylic-acid-based compounds, phospholipid, starch, calcium ions, and polyethylene glycol.
The internal environment and simulating and evaluating conditions of a polymer electrolyte fuel cell will be described later.
Patent Documents 20, 21, 22, 23, and 24 disclose that fluorine is eluted when using fluorine-based solid polymer for an electrolyte film, and it is known that a minute amount of hydrogen fluoride environment is generated in this case. Meanwhile, in a case of using a hydrocarbon polymer, it is considered that fluorine is not eluted from the electrolyte film.
Further, Patent Document 24 discloses that pH of an exhaustion liquid is about 3 in experiment. Patent Document 10 employs constant potential corrosion tests in which a potential of 1V is applied in a 50° C. aqueous sulfuric acid having a pH of 4, and Patent Documents 11, 12, 13, and 14 employ durability evaluating tests in which a potential of 0.6V is applied in a 80° C. aqueous sulfuric acid having a pH of about 2.
Patent Document 25 discloses that the driving temperature is about 80 to 100° C., for example, and Patent Documents 21 and 24 employ 80° C. as an evaluation condition. From the above, it is easily assumed that the evaluation conditions for simulating a polymer electrolyte fuel cell is an aqueous solution in which fluorine is dissolved by a solid polymer of an electrolyte film having a pH of 2 to 4, temperatures of 50 to 100° C., cell voltage changes of 0 to 1V (the voltage is 0 before power generation).
On the other hand, from the viewpoint of environment resistance of titanium, it is commonly known that titanium is dissolved by a hydrogen fluoride aqueous solution (hydrofluoric acid). Non-Patent Document 1 discloses that the addition of about 2 ppm or about 20 ppm fluorine to a pH3 aqueous sulfuric acid promotes discoloration of titanium. This discoloration is caused in a manner that titanium is dissolved and re-precipitated as an oxide on the surface, the oxide film grows, and an interference color is generated. As described above, the re-precipitated oxide is a material that inhibits contact conductivity. Accordingly, an environment in which fluorine is eluted in a fuel cell is a more harsh condition for titanium, so that durability needs to be further increased so as not to increase contact resistance.